This invention relates to internal drum photoplotters and the support on which a substrate is held during a plotting operation, and deals more particularly with a drum for such plotters and an associated method of making the same wherein the drum can be readily fabricated with a surface having a highly cylindrical shape; the invention further deals with a method of mechanically compensating for surface irregularities which may otherwise occur by precisely adjusting the orientation of the scanning means relative to the support surface.
Internal drum photoplotters utilize a section of a cylindrical surface to support a photosensitive film during a plotting operation. This surface must be as perfectly cylindrical as possible to insure the highest degree of plotting accuracy achievable on the film or photosensitive substrate supporting it. Any irregularity in this surface will cause an undesirable variance to be made in the lines projected onto the substrate. The need to provide a surface having as few irregularities as possible is to some extent also recognized in flat bed photoplotters. But, since a planar support surface is involved in this case, providing such a conforming surface is more easily effected than one having a cylindrical shape. For example, in U.S. Pat. No. 4,963,217, issued to Robert J. Pavone on Oct. 16, 1990, and being commonly assigned with the assignee of the present invention, it is disclosed to form an accurate planar surface by using a tool presenting a highly accurate flat surface to be transferred to the support surface of the platen constituted by sheet material. This is done by applying a layer of adhesive to one side of the sheet and placing it face down on its opposite side onto the tool. Subsequently, a honeycomb infrastructure is moved down into the exposed adhesive, such that when the adhesive dries, the support surface is held in the desired planar condition by the infrastructure, regardless of whether it entirely seats flushly against it. By contrast, to fabricate a section of a cylindrical surface in a drum plotter using sheet material, a more complicated process must be employed because the supporting drum base presents a surface which is curved requiring that the sheet be maintained in a curved condition during the attaching process. Further complicating this process, is the vacuum system provided in the base for dispersing vacuum evenly below the support surface so that the film can be drawn against the contour it presents. Such a system must be one in which the sheet material constituting the support surface is held rigidly in a cylindrical shape while nevertheless being capable of permitting the free flow of air communicating through the surface and thereafter through the base.
Accordingly, it is an object of the present invention to provide a drum for use in an internal drum raster photoplotter which can be readily fabricated so as to provide a support surface which is highly cylindrical for holding and supporting a substrate during a plotting operation.
It is a further object of the present invention to provide a drum support of the aforementioned type wherein the drum is made by a process allowing the support surface to be readily assembled with the highest possible degree of accuracy.
The invention further resides in a method of fixturing the scanner means of the photoplotter to account for defects or irregularities that may exist in the surface in order to minimize the need to electronically compensate for such variances in the scanning process.
Further objects and details of the invention will become apparent from the following description and appended claims.